Metal working power supply converter system and method

ABSTRACT

A power supply for welding, cutting and similar operations includes a dual two-switch forward converter. The converter has two inverter circuits coupled in parallel but controlled to provide output power in an interleaved fashion. To avoid “walking” of the circuits (which could result in different duty cycles and imbalance of the load sharing), control signals are determined and applied to a first of the inverter circuits, and “on” times of the first circuit is monitored, such as by augmenting a counter to determine the number of clock cycles the first circuit is “on”. The same duration is then used for commanding output from the second inverter circuit. The duty cycles of both circuits is thus ensured to be the same regardless of changes in the total output power.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation patent application of U.S.Non-Provisional application Ser. No. 13/925,579, entitled “Metal WorkingPower Supply Converter System and Method”, filed Jun. 24, 2013, which isherein incorporated by reference in its entirety for all purposes.

BACKGROUND

The invention relates generally to a field of welding systems andsystems for performing metal working operations, such as gas metal arcwelding (GMAW). More particular, the disclosure relates to innovationsin the control of power supplies and converter circuitry used in suchsystems.

Many applications exist for welding and cutting systems used to join twoor more workpieces to one another, or to cut workpieces. Theseapplications exist throughout industry, but also for construction, shipbuilding, maintenance, and so forth. In arc welding systems, electricalpower is converted to a form useful for a welding process, and the powerprovides voltage and current necessary to establish and maintain arcsbetween an electrode and a workpiece. Plasma cutting and similaroperations also require conditioned power adapted for the specificprocess. In gas metal arc welding (GMAW), the arc is established betweenan electrode that is advanced towards the workpiece and of the workpieceitself. The electrode is consumed insomuch as it is added to the weld asthe weld puddle advances during the operation.

In welding and cutting power supplies, power electronic circuitry iscommonly switched on and off to control the desired power output usedfor the process. The switching is commonly performed by pulse widthmodulation (PWM) signals applied to the gates of power electronicswitches of converter circuits within the supplies. Conventional systemsutilize a single inverter in this converter circuitry, along with aninductor to smooth the output waveform. However, inductors in suchsystems can be quite large, implying additional cost and weight.Improvements in these systems have included the use of two or moreinverters or converters which are switched to provide the desiredoutput. The ripple amplitude of the output current is thereby reduced,consequently reducing the size of the output inductor.

One problem in such systems exists insomuch as independently controlledinverter circuits can become unbalanced during operation. The result canbe a “walking” deviation in the duty cycles of the two inverters, inwhich one of the inverters will tend to progressively take on most ofthe load, causing thermal imbalance, undermining the benefits obtainedby the use of multiple inverters. Moreover, current systems may notaccount for the magnetic dynamics of transformer circuits in the powerconverters. With common assumptions as to magnetizing and demagnetizingof the transformers, less than the full potential power output isobtained when a PWM duty cycle of 50% is treated as a limit for each ofthe unipolar or single ended inverter circuits.

Improvements are therefore needed in the field that would allow foravoiding drawbacks such as those described above.

BRIEF DESCRIPTION

The present disclosure sets forth certain solutions that maybeimplemented in welding and similar power supplies and that are designedto respond to such needs. In accordance with certain aspects of thepresent disclosure, a welding or cutting power supply system includes aconverter circuit comprising first and second solid state switchingcircuits each having two power electronic switches for producing outputpower at a controllable level suitable for a welding or cuttingoperation, the first and second solid state switching circuits beingelectrically coupled in to provide combined output. An output inductanceis coupled to outputs of both the first and second solid state switchingcircuits. Control circuitry is coupled to the first and second solidstate switching circuits and configured to provide PWM control signalsfor controlling states of the power electronic switches of the switchingcircuits to maintain desired loading of the switching circuitsindependent of the level of the output power. For each cycle of theswitching circuits the control circuitry is configured to apply PWMcontrol signals to the first switching circuit based upon a desired dutycycle, to determine the duration of the PWM control signals applied tothe first switching circuit, and to apply PWM control signals to thesecond converter based upon the determined duration withoutre-determining the duty cycle for the second switching circuit.

In accordance with other aspects, a welding or cutting system comprisesa power supply comprising first and second solid state switchingcircuits each having two power electronic switches for producing outputpower at a controllable level suitable for a welding or cuttingoperation, the first and second solid state switching circuits beingelectrically coupled in parallel, and an output inductor coupled tooutputs of both the first and second solid state switching circuits.Control circuitry is coupled to the first and second solid stateswitching circuits and configured to determine PWM control signals forthe first switching circuit based upon a desired duty cycle, todetermine the duration of the PWM control signals of the first switchingcircuit, and to determine PWM control signals to the second converterbased upon the determined duration without re-determining the duty cyclefor the second switching circuit.

In accordance with still further aspects, a welding or cutting method,comprises applying PWM control signals to a first switching circuit of apower converter comprising at least two switching circuits operated inan interleaved switching pattern, the control signals of the firstswitching circuit being based upon a desired duty cycle to produce awelding or cutting output, and determining a duration of the PWM controlsignals applied to the first switching circuit. PWM control signals arethen applied to a second switching circuit of the power converter for aduration based upon the determined PWM duration.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical representation of an exemplary GMAW weldingsystem incorporating power conversion circuitry in accordance with thepresent disclosure;

FIG. 2 is a diagrammatical representation of exemplary components of thepower conversion circuitry;

FIG. 3 is an electrical schematic of a currently contemplated dualtwo-switch power converter utilizing aspects of the present disclosure;

FIG. 4 is a flow chart illustrating exemplary logic for providingcontrol signals to the two converters of the circuit of FIG. 3;

FIGS. 5-10 are graphical representations of exemplary control and outputwaveforms exemplifying the control of the two converters in accordancewith the present techniques;

FIGS. 11-14 are graphical representations of switched control signalsfor more fully utilizing the capabilities of the circuitry; and

FIGS. 15-19 are graphical representations of exemplary voltage andcurrent waveforms of the type obtainable through use of the controlscheme outlined in FIGS. 11-14.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary welding system 10 in the form of a GMAWsystem, and in particular a MIG welding system. In this exemplary systema power supply 12 receives and converts power that is applied to a wirefeeder 14. The wire feeder delivers the power to a welding torch 16 forcompleting a welding operation. Although the present disclosure usedthis GMAW power supply as an exemplary basis for presentation of the newcircuitry and control techniques, it should be understood that the sameteachings may be applied to power supplies used for other weldingprocesses, as well as for other metal working processes, such as plasmacutting.

The power supply 12 receives input power 18 from any suitable source,such as the power grid, an engine generator set, hybrid power supplies,fuel cells, batteries, or a combination of these. Power conversioncircuitry 20 converts the power to a form suitable for a welding (orother metal working) process. The power supply may be designed to carryout multiple different welding processes that can be selected by anoperator, and the power conversion circuitry includes components, suchas solid state switches discussed below, that allow for power conversionin accordance with the desired process. Control and processing circuitry22 is coupled to the power conversion circuitry 20 and controls theoperation of the power conversion circuitry during the selected process.For example, the control and processing circuitry 22 may provide signalsthat regulate the conductive states of solid state switches within thepower conversion circuitry to produce the desired output power, as alsodiscussed below. In many applications the control and processingcircuitry will include one or more digital processors or microprocessorswith associated memory to store and carry out the processes available onthe power supply. Such processes may include constant voltage (CV)processes, constant current (CC) processes, pulsed processes, cuttingprocesses, and so forth. The processes and other welding parameters maybe selected via an operator interface 24 that is coupled to the controland processing circuitry 22. The power supply may further includecircuitry that allows for communications with remote or networkedcomponents and systems, illustrated as data/network interface 26 inFIG. 1. Such circuitry may allow for monitoring of welding operations,logging of weld data, downloading or configuration of new processes andupdates to processes, and so forth. Finally, the power supply willsometimes include removable memory 28 that may be used for storingprocesses, process parameters, system updates, and any suitable data.

Power and data may be transferred from the power supply 12 to the wirefeeder 14 via one or more cables or cable bundles 30. The wire feederitself comprises a drive control circuitry 32 that regulates theoperation of a drive assembly 34. The drive assembly 34 contacts andfeeds a wire electrode 36 to the welding operation. The wire electrodeis typically stored on a spool 38 within the wire feeder. The wirefeeder may also include one or more gas valves for providing shieldinggas for a welding operation. Finally, an operator interface 42 may allowcertain parameters of the wire feeder to be selected, such as wire feedspeed. The power supply and wire feeder may operate in coordination sothat wire and gas resources are fed to the welding operation when poweris provided for welding at the initiative of the welding operator (e.g.,via a control on the torch). In some embodiments the power supply andwire feeder may be integrated into a single package. The wire and gasresources are provided via a weld cable 44 coupled to the torch. Asecond or work cable 46 is typically clamped or coupled in some mannerto a workpiece 48 for completing the electrical circuit. The fullcircuit is completed during the welding operation by an arc as indicatedat reference numeral 50.

FIG. 2 illustrates exemplary functional components for the powerconversion circuitry 20. As shown in FIG. 2, this circuitry may includean electromagnetic interference filter 52. Such circuitry will typicallyprovide an inductive-capacitive filter, and may include protectivedevices such as varistors. An input rectifier 54 converts AC power to DCpower, and will typically include a full bridge rectifier. The DC poweris placed on a DC bus, and the circuitry may include bus capacitors tosmooth the ripple of the DC waveform on the DC bus. A power factorcorrection boost circuit 56 may allow for power factor correction withthe input power. Some systems may not include this boost circuit. Whereprovided, this may be a switched circuit that steps up the DC busvoltage to a desired level, such as 440V. An isolated converter 58,which is discussed in greater detail below, includes, in a presentlycontemplated embodiment, a dual two-switch interleaved forward converterthat itself includes an output rectifier and an output inductor.Exemplary electrical components for this circuitry, along with theiroperation and control are described below. The output of the converter58 may be provided to the welding (or other) application. In a presentlycontemplated embodiment, the PFC boost circuitry 56 and the powerconverter 58 are provided with control circuits 62 and 64 to regulatetheir operation. Such control circuits will typically serve to providegate signals to solid state switches within the power converters inaccordance with the particular welding (or cutting) regime, the poweroutput needs, and so forth.

FIG. 3 illustrates an exemplary interleaved forward converter of thetype mentioned above. The forward converter 66 in this embodiment is adual two-switch interleaved forward converter that comprises first andsecond inverter circuits 68 and 70 coupled generally in parallel butcontrolled in an interleaved fashion. The outputs of the two circuitsare joined as illustrated, with an output inductor 72 common to bothcircuits to smooth the output waveform. It should be noted that in somesystems, particularly in welding systems, a welding cable may providesufficient parasitic inductance such that the separate inductor may notbe required, particularly where the frequency of the output waveform issufficiently high. More particularly, the first inverter circuit 68comprises two solid state switches 74 and 76, which may typically beinsulated gate bipolar transistors with freewheel diodes 78 and 80,respectively. Application of pulsed control signals to the gates ofthese devices allows for creation of an output waveform that is appliedto a transformer 82. Analogous components are provided in the secondinverter circuit 70 in the form of switches 84 and 86, diodes 88 and 90,and transformer 92. Each inverter circuit includes diodes 94 and 96which form an output rectifying circuit, that serve as rectifying andfreewheeling functions, respectively.

In operation, the two inverters are controlled by pulse width modulatedsignals that cause the circuits to alternatively produce output that iseffectively summed. To ensure that both circuits produce equal output,and that the load is shared, the current control scheme effectivelygenerates and provides pulse width modulated gate drive signals to afirst of the inverter circuits, monitors the duration of the “on” stateof the first circuit (the period during which the circuit is controlledto produce output power), then pulses the second inverter circuit “on”for a similar period to ensure the same duty cycle.

This process is summarized generally in FIG. 4. The control logic 98 maybe represented as beginning with activation of forward converter one(corresponding to inverter circuit 68 in FIG. 3) as indicated at step100. The first forward converter is activated to provide an output for adesired period, depending upon the needs of the welding (or other)operation. In this basic control scenario, each forward converter isallowed to be placed in an “on” output state for one half of the time.Thus, as indicated at step 102, the “on” period of the first converteris monitored or accumulated, such as by augmenting a counter for clockcycles during which the first converter is “on”. Other techniques maycertainly be employed for this purpose, including other methods ofmeasuring time, storing durations of gate drive signals, and so forth.As indicated at step 104, it is then determined whether the maximumallowed period for activation of the first converter has been reached.In many embodiments, this period may be equal to a 50% duty cycle orless, although periods of greater than 50% may be used, as discussedbelow. Once the maximum period for activation of the first converter isreached, then, the second forward converter is activated as indicated atstep 106. The second forward converter is activated (e.g., gate drivesignals are provided to the solid state switches) for a period equal tothat during which the first forward converter was “on”. The query block108 in FIG. 4 illustrates that the activation of the second forwardconverter continues until the period is equal (e.g., the same number ofclock cycles counted for the first converter has expired for thesecond). At step 110 it is determined whether the maximum activationtime for the second converter is reached. Once the results of bothblocks 108 and 110 are affirmative, the second forward converter isdeactivated and the first forward converter may be reactivated

FIGS. 5-10 are graphical illustrations of PWM control signals that maybe applied to the switches of each of the converters 68 and 70, as wellas output waveforms for currents of each. As illustrated in FIG. 5,control signals 118 for the first converter 68 are applied in pulsesthat may approach a 50% duty cycle. (As described with respect to FIGS.11-14, this duty cycle, may, in fact, exceed 50%.) Thus, “on” periods120 are those in which the solid state switches 74 and 76 receive gatedrive signals to place them in a conductive state (alternatively) and“off” periods 122 are periods during which the signals to the switchesmaintain them in a non-conductive state. The ratio of the “on” time tothe “off” time, designated by reference numerals 124 and 126,respectively, corresponds to the duty cycle of the first converter 68.Similarly, as depicted in FIG. 6, PWM control signals to the secondconverter, designated generally by reference numeral 128, include “on”periods 130 and “off” periods 132 in which the switches 84 and 86(alternatively) are placed in conductive and non-conductive states,respectively. Here again, as will be appreciated by those skilled in theart, due to the inverter circuitry design the inverter switches aretypically switched alternatively. The resulting “on” time 134 and “off”time 136 provide the duty cycle for the second converter 170. Ideally,these duty cycles will be identical. When combined, then, as illustratedin FIG. 7, the interleaved switching of converters 168 and 170 produce acombined duty cycle with “off” periods 140 between the alternatingoutputs provided by the respective converters.

In the present embodiment, as described with respect to FIG. 4, each ofthe converters is available to be placed in a conductive state toprovide output power at a specific point in time. Thus, the “on” state120 in FIG. 5 for the first converter begins at the left axis in thatfigure, and the next available point of initiation for placing the firstconverter in a “on” state begins at the period resulting from the sum ofperiods 124 and 126 (i.e., the switching period). The second convertersimilarly is available to be placed in an “on” state at a specific timethat is 180° displaced from that of the first converter (i.e., at apoint at one half of the switching period). That is, given the totalperiod (period 124+126), which is equal for both converters, the secondconverter is available to be switched into the “on” or output state at atime half way through the total switching period.

FIG. 8 illustrates the output of the first converter in a currentwaveform. As illustrated, the current 142 will rise as indicated byreference numeral 144 so long as the converter is being PWM'ed foroutput, and will then decline following shut-off of the output of thefirst converter as indicated by reference numeral 146. The current 148of the second converter, illustrated in FIG. 9, will similarly rise asindicated by reference numeral 150 when this converter is placed in the“on” or output state, and will decline as indicated by reference numeral152 when “off”. The combined currents, then, illustrated in FIG. 10 andreferred generally by reference numeral 154, will be the combination ofthese waveforms exhibiting a reduced ripple current and generallysmoothed output at twice the PWM frequency of either converter, asindicated by trace 156.

As described above, the duty cycles of the two converters are maintainedequal during this process. To summarize, control of the first converteris based upon the process selected, the output power parameters desired,and so forth. In this way the desired “on” time and waveformcharacteristics of the first converter are determined, and PWM controlsignals are applied to the gates of the solid state switches of thefirst converter for a desired time. This time, designated by referencenumeral 124 in FIG. 5, is counted, such as by augmenting a counter C (asshown in FIG. 2) within the control circuitry logic to count the numberof clock cycles (or any other reference cycle) during which the firstconverter is commanded to provide output. Subsequently, rather thanre-determine the required output for the second converter, the secondconverter PWM control signals are simply applied for an equal time basedupon the counter accumulation which is referenced from the immediatelypreceding control signals applied to the first converter. The twoconverters are thus ensured with having the same duty cycle regardlessof changes in the duty cycle applied to the first converter.

As mentioned above, the present disclosure also provides a mechanism forenhancing the power output of the power converter circuitry describedabove. In particular, the PWM control signals that command theindividual interleaved converters to provide output may be extendedbeyond the 50% duty cycle range (as discussed below). In conventionalsystems, it is typically believed that when implementing an invertertopology consisting of a single-ended converter such as a forwardconverter, the maximum pulse width to the primary of the downstreamtransformer should not exceed 50%. This traditional treatment is likelythe result of the belief that as much time is required to discharge themagnetizing inductance of the transformer as the time required to chargeit. Under no-load conditions, this 50% limit generally holds truebecause the magnetizing inductance charges for the entire time that theprimary pulse (“on” period) is applied. However, when the converter isrunning in constant conduction mode (CCM) and there is load currentflowing, there is little or no magnetizing current flowing in thetransformer until the current in the leakage inductance matches thereflected load current. Under high load conditions, it may requireseveral microseconds to “charge” the leakage inductance. Also, duringthis leakage inductance charging time, no secondary voltage is appliedto the transformer secondary, which implies that no power is beingtransformed to the load. This phenomenon may result in an “effective”pulse width where the secondary pulse width is equal to the primarypulse width minus the leakage inductance charge time (which is afunction of the load current). When the primary side pulse width isapproaching its maximum, say 12.5 microseconds for a 40 KHz forwardconverter, the secondary pulse width can be substantially less, say onthe order of 9 microseconds. This results in an effective pulse widthduty of 36% and not 50%. Consequently, if the output voltage is equal tothe input voltage times the PWM duty cycle, a limit of the output meanvoltage that can be achieved is below the typical 50% limit.

In accordance with the present disclosure, a maximum primary pulse widthmay be allowed to extend beyond the 50% limit based upon the amount ofload current. This extension results from a realization that half of theleakage inductance charge time could be added to the pulse width andwould still provide adequate time for the magnetizing inductance todischarge without changing the overall cycle period. By way of example,this could raise the effective secondary pulse width to 43% or anadditional 7% load voltage without requiring a change in the turns ratioof the transformer. This technique is generally illustrated in FIGS.11-14.

As shown in FIG. 11, the PWM drive signals 158 for the first convertercomprise PWM commands 160 that place the converter in an “on” state,followed by periods 162 in which the converter is “off”. An extension164 of the “on” period beyond the normal 50% limit 166 is thus providedto a new switching time 168 at which the converter is switched “off”.The PWM control signals 170 for the second converter, illustrated inFIG. 12, may be similarly extended. Thus, PWM control signals to placethe converter in a “on” state 172 may begin at the normal time in theoverall cycle at which they would have otherwise begun, indicated byreference numeral 174. However, a similar extension in the control maybe made that is equal to that made for the first converter. Thus, the“on” period of the first converter, indicated by reference numeral 176remains equal to that for the second converter, indicated by referencenumeral 178. Due to the magnetizing and demagnetizing dynamics of thetransformers, a higher mean voltage output is obtained than would beavailable if the strict 50% duty cycle for each interleaved converterwere held.

Implementation of the approach may follow the same logic as thatdescribed above. That is, the PWM duty cycle of the first converter maybe determined based upon the process, power output requirements, and soforth. The actual “on” period for the first converter is then detectedand recorded (e.g., by augmenting a counter based upon clock cyclesduring the “on” period), and a second converter is placed in the “on”state for an equal time.

Where processing capabilities or control logic does not permit suchoperation (e.g., where it is preferable to determine the full “on”period for the second converter prior to switching it to the “on”state), a delayed approach illustrated in FIGS. 13 and 14 may beadopted. In this approach, the control signals 180 for the firstconverter will include a series of “on” PWM inputs 182, 184, 186, and soforth. However, the control signals 188 for the second converter may bedelayed one cycle to follow the first converter. That is, the controlsignals 188 for the second converter will include PWM commands 190, 192,194, 196, and so forth. The delay will result in a one-cycle shift ofthe PWM command of the first converter in its application to the secondconverter, as indicated for the “n-1” signals 184 and 194, and the “n”commands 186 and 196 illustrated in both FIGS. 13 and 14. This delayallows the full duration of the “on” period of the second converter tobe known before it is applied (in the next switching period).

FIGS. 15-19 are graphical representations of exemplary voltage andcurrent waveforms of the type obtainable through use of the controlscheme outlined in FIGS. 11-14. In particular, FIG. 15 representstransformer primary control voltages 200 over time. As illustrated, whenthe converters are switched “on”, the voltage rises as indicated byblocks 202, and when “off”, the voltage drops as indicated by referencenumeral 204. As shown in FIG. 16, then, the no-load magnetizing current206, then, rises, as indicated by reference numeral 208 when thetransformer primary voltage is high, and declines, as indicated byreference numeral 210 when the voltage is low. As shown in FIG. 17, thefull load primary current 212 rises, as indicated by reference numeral214 during an initial period 216. This initial rise corresponds to theleakage inductance current charge only. Then, as indicated by referencenumeral 218, the current rises at a lower rate during a period 220, dueto the effective secondary voltage and inductance. The decline in thefull load primary current, as indicated by reference numeral 222, occursfollowing removal of the primary voltage.

The full load secondary voltage is illustrated in FIG. 18. The voltageis shown as rising, as indicated by reference numerals 226 when theprimary voltage is high, but with the “on” level shifted due to theleakage inductance of the transformer. When the primary voltage isremoved, the secondary voltage falls, as indicated by reference numeral228. The full load magnetizing current 230 is illustrated in FIG. 19.Here, the current rises, as indicated by reference numeral 232 when thesecondary voltage is high, and declines, as indicated by referencenumeral 234 when the secondary voltage is low. An unutilized portion ofthe pulse width then results, as indicated by reference numeral 236. Thetechniques described above allow for more full utilization of the pulsewidth during such periods by extending the command PWM signals beyondthe 50% level, as discussed above.

Various modification and variations of the circuitry, systems andtechniques described above may be envisaged. For example, while twoinverters or converters have been described, the same techniques may beused in systems with more than two such circuits connected to providecommon output. Also, while forward converters are illustrated anddescribed, other converter types and applications may be used. Further,while digital, discrete techniques are described for determiningswitching of the converters, analog and hybrid circuitry may also beemployed for this purpose. Finally, while equal time periods forswitching of the converters are described, the same or similartechniques may be used for controlling loading (and heating) of thecircuits by dissimilar switching periods (e.g., based on adding orsubtracting from the counts of clock pulses of a first converter whencontrolling a second converter).

While in the foregoing discussion and examples two converters wereutilized, it should be noted that the extended period control techniquesdiscussed may be used in systems utilizing a single converter as well.While such converters (e.g., single ended converters) have typicallybeen utilized with a 50% duty cycle limit, it has been determined thatusing the present teachings an extended duty cycle (i.e., not limited toa maximum of 50%) can be obtained. Here again, the present techniquesmay be used with systems having more than two converters as well.

It should also be noted that the PWM duty cycle implemented willtypically be changed during operation of the system, and that theparticularly duty cycle at any time may be dependent upon the load, suchas the power drawn by the welding or cutting operation in the presentcontext. Such control may be based on monitoring power and/or currentdrawn by the load. In one presently contemplated embodiment, forexample, the PWM duty cycle might be extended by a desired number ofclock pulses (or any other desired standard period) for each amp ofoutput current drawn by the load. It may also be based on meteringand/or determining a time between a point when the transformer primarycurrent falls to zero and a start of a subsequent switching period, asshown in the foregoing graphical illustrations.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A welding or cutting power supply system, comprising: a convertercircuit comprising first and second solid state switching circuits eachhaving two power electronic switches for producing output power at acontrollable level suitable for a welding or cutting operation, thefirst and second solid state switching circuits being electricallycoupled to provide combined output; an output inductance coupled tooutputs of both the first and second solid state switching circuits; andcontrol circuitry coupled to the first and second solid state switchingcircuits and configured to provide PWM control signals for controllingstates of the power electronic switches of the first and second solidstate switching circuits to maintain desired loading of the first andsecond solid state switching circuits independent of a level of theoutput power, wherein for each cycle of the first and second solid stateswitching circuits the control circuitry is configured to apply PWMcontrol signals to the first solid state switching circuit based upon adesired duty cycle, to determine a duration of the PWM control signalsapplied to the first solid state switching circuit by counting a numberof clock cycles that the PWM control signals are applied to the firstsolid state switching circuit, and to apply PWM control signals basedupon the determined duration without re-determining a duty cycle for thesecond solid state switching circuit.
 2. The welding or cutting powersupply system of claim 1, wherein the number of clock cycles is countedby augmenting a counter implemented by the control circuitry.
 3. Thewelding or cutting power supply system of claim 1, wherein a duty cycleof each solid state switching circuit is limited to 50% or less.
 4. Thewelding or cutting power supply system of claim 1, wherein a duty cycleof each solid state switching circuit is generally equal.
 5. The weldingor cutting power supply system of claim 1, wherein at least two solidstate switches of each solid state switching circuit are coupled acrossa common DC bus.
 6. The welding or cutting power supply system of claim5, wherein each solid state switching circuit comprises an outputtransformer and a freewheeling diode across the common DC bus.
 7. Thewelding or cutting power supply system of claim 1, wherein each solidstate switching circuit comprises a transformer between the respectivesolid state switches of the first and second solid state switchingcircuits and an output rectifying circuit.
 8. The welding or cuttingpower supply system of claim 7, wherein each solid state switchingcircuit comprises a rectifier between a respective transformer and theoutput inductance.
 9. The welding or cutting power supply system ofclaim 1, wherein the welding or cutting power supply system producesoutput power suitable for a GMAW welding process.
 10. A welding orcutting system, comprising: a power supply comprising first and secondsolid state switching circuits each having two power electronic switchesfor producing output power at a controllable level suitable for awelding or cutting operation, the first and second solid state switchingcircuits being electrically coupled in parallel; an output inductancecoupled to outputs of both the first and second solid state switchingcircuits; and control circuitry coupled to the first and second solidstate switching circuits and configured to determine PWM control signalsfor the first solid state switching circuit based upon a desired dutycycle, to determine a duration of the PWM control signals of the firstsolid state switching circuit by counting a number of clock cycles thatthe PWM control signals are applied to the first solid state switchingcircuit, and to determine PWM control signals based upon the determinedduration without re-determining a duty cycle for the second solid stateswitching circuit.
 11. The welding or cutting system of claim 10,wherein the number of clock cycles is counted by augmenting a counterimplemented by the control circuitry.
 12. The welding or cutting systemof claim 10, wherein a duty cycle of each solid state switching circuitis limited to 50%.
 13. The welding or cutting system of claim 10,wherein a duty cycle of each solid state switching circuit is generallyequal.
 14. The welding or cutting system of claim 10, wherein at leasttwo solid state switches of each solid state switching circuit arecoupled across a common DC bus.
 15. The welding or cutting system ofclaim 14, wherein each solid state switching circuit comprises an outputtransformer and a freewheeling diode across the common DC bus.
 16. Thewelding or cutting system of claim 10, wherein each solid stateswitching circuit comprises a transformer between the respective solidstate switches of the first and second solid state switching circuitsand an output rectifying circuit.
 17. The welding or cutting system ofclaim 16, wherein each solid state switching circuit comprises arectifier between a respective transformer and the output inductance.18. The welding or cutting system of claim 10, wherein the welding orcutting system produces output power suitable for a GMAW weldingprocess.
 19. A welding or cutting method, comprising: applying PWMcontrol signals to a first switching circuit of a power convertercomprising at least two switching circuits operated in a desiredswitching pattern, the PWM control signals of the first switchingcircuit being based upon a desired duty cycle to produce a welding orcutting output; determining a duration of the PWM control signalsapplied to the first switching circuit by counting a number of clockcycles during which the PWM control signals are applied; and applyingPWM control signals to a second switching circuit of the power converterfor a duration based upon the determined duration of the PWM controlsignals.
 20. The method of claim 19, wherein the PWM control signalsapplied to the second switching circuit are not determined separatelyother than by determining the duration based upon the duration of thePWM control signals applied to the first switching circuit.
 21. Themethod of claim 19, comprising switching the switching circuits in aninterleaved switching pattern.
 22. The method of claim 21, comprisingindependently transforming and rectifying the outputs of all theswitching circuits.